Reciprocal drivers used in loudspeakers typically include a chassis, which forms the rigid mechanical framework for the driver, a vibrating diaphragm, which is driven axially by means of electromagnetic induction forces generated by alternating current, and a suspension element surrounding the diaphragm and elastically coupling it to the chassis. It is paramount that the movement of the diaphragm is precisely and accurately controlled, which is a matter of suspension element design. Ideally, the movement of the diaphragm is linear, or in other words, the diaphragm motion in the axial direction is directly proportional to the magnitude of the alternating current that is applied to the driver. If the movement of the diaphragm is non-linear, then the sound becomes distorted.
Generally speaking, the aim is to provide a progressive suspension element with fairly constant stiffness for small displacements, and a rapidly increasing stiffness for large displacements. Thus, an ideal progressive suspension element will add low amounts of non-linearity (distortion) to the motion of the diaphragm for small displacements whilst also protecting the driver from damage during large excursions.
The surrounding suspension element of a loudspeaker driver is easier to design when the shape of the suspension element is essentially round in relation to the direction of movement of the driver diaphragm. In such a configuration, there is axial-symmetry and the force exerted by the suspension element (restoring the diaphragm to its rest position) is usually equal and symmetrical at all locations around the perimeter of the suspension element. Typically, when the shape of the suspension element is essentially round, the cross-sectional profile of the suspension element has the same geometry all the way around the perimeter of the suspension element.
The suspension properties of the suspension element are typically expressed by means of stiffness profile, i.e., a chart that plots the stiffness of the suspension versus the displacement of the diaphragm. For a low distortion driver, the stiffness should be fairly even for small displacements and the stiffness should be fairly symmetrical, i.e., fairly equal stiffness values for positive and negative displacements.
Designing the suspension of the diaphragm becomes more complicated when the geometry of the diaphragm has not only curved sections but also straight sections. More precisely, suspension design is more challenging for diaphragms having straight sections joined together by curves, i.e., a “stadium shape”. Such drivers generally suffer from uneven distribution of the forces exerted by the suspension element for restoring the diaphragm to its rest position. The stiffness profiles of such drivers can be very non-linear and the progressive suspension that should prevent over-excursion of the diaphragm to prevent damage is not always functioning as it should. This sort of non-linearity may appear as distortion in the output curve of the loudspeaker.
It is therefore an aim to provide a loudspeaker driver not suffering from high levels of distortion caused by the non-linear stiffness commonly found with drivers that utilize progressive suspension elements.
It is a particular aim of the invention to provide a suspension element for a vibrating diaphragm, which has a geometry featuring two parallel opposing straight sections and two opposing curved sections connecting the two straight sections, and which diaphragm would have a more idealized stiffness profile with a linear (low distortion) diaphragm motion for small displacements and a rapidly increasing stiffness for high displacements to prevent driver damage resulting from over excursion. It is also an aim of the present invention to re-distribute the restoring forces exerted by the suspension element onto the diaphragm in a way that reduces problems caused by standing wave resonance patterns which add unwanted color to the sound. By combining tangential stress relief measures with the re-distribution of the suspension element's restoring forces it is hoped that the linear excursion range can be increased further than conventional speaker designs.